MPLS Traffic Engineering: RSVP Hello State Timer

First Published: August 2, 2004

Last Updated: February 27, 2009

The MPLS Traffic Engineering: RSVP Hello State Timer feature detects when a neighbor is down and quickly triggers a state timeout, which frees resources such as bandwidth that can be reused by other label switched paths (LSPs).

RSVP hellos can be used to detect when a neighboring node is down. The hello state timer then triggers a state timeout. As a result, network convergence time is reduced, and nodes can forward traffic on alternate paths or assist in stateful switchover (SSO) operation.

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Information About MPLS Traffic Engineering: RSVP Hello State Timer

Hellos for State Timeout

When RSVP signals a TE LSP and there is a failure somewhere along the path, the failure can remain undetected for as long as two minutes. During this time, bandwidth is held by the nonfunctioning LSP on the nodes downstream from the point of failure along the path with the state intact. If this bandwidth is needed by headend tunnels to signal or resignal LSPs, tunnels may fail to come up for several minutes thereby negatively affecting convergence time.

Hellos enable RSVP nodes to detect when a neighboring node is not reachable. After a certain number of intervals, hellos notice that a neighbor is not responding and delete its state. This action frees the node's resources to be reused by other LSPs.

Hellos must be configured both globally on the router and on the specific interface to be operational.

Nonfast-Reroutable TE LSP

Assume that the link between Router 1 and Router 2 fails. This type of problem can be detected by various means including interface failure, Interior Gateway Protocol (IGP) (Open Shortest Path First (OSPF) or Intermediate System-to-Intermediate System (IS-IS)), and RSVP hellos. However, sometimes interface failure cannot be detected; for example, when Router 1 and Router 2 are interconnected through a Layer 2 switch. The IGP may be slow detecting the failure. Or there may be no IGP running between Router 1 and Router 2; for example, between two Autonomous System Boundary Routers (ASBRs) interconnecting two autonomous systems.

If hellos were running between Router 1 and Router 2, each router would notice that communication was lost and time out the state immediately.

Router 2 sends a delayed PathTear message to Router 3 so that the state can be deleted on all nodes thereby speeding up the convergence time.

Note The PathTear message is delayed one second because on some platforms data is being forwarded even after the control plane is down.

Hello Instance

A hello instance implements RSVP hellos for a given router interface address and a remote IP address. A hello instance is expensive because of the large number of hello requests that are sent and the strains they put on the router resources. Therefore, you should create a hello instance only when it is needed to time out state and delete the hello instance when it is no longer necessary.

Fast-Reroutable TE LSP with Backup Tunnel

Figure 2 shows a fast reroutable TE LSP with a backup tunnel from Router1 to Router 2 to Router 3.

Figure 2 Fast Reroutable TE LSP with Backup Tunnel

This TE LSP has a backup tunnel from Router 1 to Router 3 protecting the fast reroutable TE LSP against a failure in the Router 1 to Router 2 link and node Router 2. However, assume that a failure occurs in the link connecting Router 1 to Router 2. If hellos were running between Router 1 and Router 2, the routers would notice that the link is down, but would not time out the state. Router 2 notices the failure, but cannot time out the TE LSP because Router 2 may be a merge point, or another downstream node may be a merge point. Router 1 notices the failure and switches to the backup LSP; however, Router 1 cannot time out the state either.

Note A hello instance is not created in the preceding scenario because the neighbor is down and the hello instance cannot take action.

Fast-Reroutable TE LSP Without Backup Tunnel

On a fast-reroutable TE LSP with no backup tunnel, a hello instance can be created with the neighbor downstream (next hop (NHOP)). On a nonfast-reroutable TE LSP, a hello instance can be created with the neighbor downstream (NHOP) and the neighbor upstream (previous hop (PHOP)). This is in addition to the existing hellos for Fast Reroute.

Note If both Fast Reroute and hellos for state timeout hello instances are needed on the same link, only one hello instance is created. It will have the Fast Reroute configuration including interval, missed refreshes, and differentiated services code point (DSCP). When a neighbor is down, Fast Reroute and the hello state timer take action.

Figure 3 shows a fast-reroutable TE LSP. without a backup tunnel, from Router 1 (the point of local repair (PLR)), to Router 2 to Router 3.

Figure 3 Fast Reroutable TE LSP Without Backup Tunnel

Assume that a failure occurs in the link connecting Router 1 to Router 3. Router 1 can time out the state for the TE LSP because Router 1 knows there is no backup tunnel. However, Router 2 cannot time out the state because Router 2 does not know whether a backup tunnel exists. Also, Router 2 may be a merge point, and therefore cannot time out the state.

Note A hello instance is not created in the preceding scenario because the neighbor is down and the hello instance cannot take action.

How to Configure MPLS Traffic Engineering: RSVP Hello State Timer

Note The following tasks also enable Fast Reroute; however, this section focuses on the RSVP hello state timer.

MPLS Traffic Engineering: RSVP Hello State Timer: Example

In the following example, the hello state timer is enabled globally and on an interface. Related parameters, including a DSCP value, a refresh interval, and a missed refresh limit, are set on an interface.

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The MPLS Traffic Engineering: RSVP Hello State Timer feature detects when a neighbor is down and quickly triggers a state timeout, which frees resources such as bandwidth that can be reused by other label switched paths (LSPs).

In 12.0(29)S, this feature was introduced.

This feature was integrated into Cisco IOS Release 12.2(33)SRA.

This feature was integrated into Cisco IOS Release 12.2(33)SXH.

This feature was integrated into Cisco IOS Release 12.4(20)T.

Glossary

autonomous system—A collection of networks that share the same routing protocol and that are under the same system administration.

ASBR—autonomous system boundary router. A router that connects and exchanges information between two or more autonomous systems.

backup tunnel—An MPLS traffic engineering tunnel used to protect other (primary) tunnel traffic when a link or node failure occurs.

DSCP—differentiated services code point. Six bits in the IP header, as defined by the IETF. These bits determine the class of service provided to the IP packet.

Fast Reroute—A mechanism for protecting MPLS traffic engineering (TE) LSPs from link and node failure by locally repairing the LSPs at the point of failure, allowing data to continue to flow on them while their headend routers attempt to establish end-to-end LSPs to replace them. FRR locally repairs the protected LSPs by rerouting them over backup tunnels that bypass failed links or nodes.

graceful restart—A process for helping a neighboring Route Processor restart after a node failure has occurred.

headend—The router that originates and maintains a given LSP. This is the first router in the LSP's path.

IGP—Interior Gateway Protocol. Internet protocol used to exchange routing information within an autonomous system. Examples of common Internet IGPs include IGRP, OSPF, and RIP.

LDP—Label Distribution Protocol. The protocol that supports MPLS hop-by-hop forwarding by distributing bindings between labels and network prefixes. The Cisco proprietary version of this protocol is the Tag Distribution Protocol (TDP).

LSP—label switched path is a configured connection between two routers, in which MPLS is used to carry packets. The LSP is created by the concatenation of one or more label-switched hops, allowing a packet to be forwarded by swapping labels from one MPLS node to another MPLS node.

merge point—The backup tunnel's tail.

MPLS—Multiprotocol Label Switching. A method for forwarding packets (frames) through a network. MPLS enables routers at the edge of a network to apply labels to packets (frames). ATM switches or existing routers in the network core can switch packets according to the labels.

RSVP—Resource Reservation Protocol. A protocol that supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature (bandwidth, jitter, maximum burst, and so on) of the packet streams they want to receive.

state—Information that a router must maintain about each LSP. The information is used for rerouting tunnels.

tailend—The router upon which an LSP is terminated. This is the last router in the LSP's path.

TE—traffic engineering. The techniques and processes used to cause routed traffic to travel through the network on a path other than the one that would have been chosen if standard routing methods had been used.

topology—The physical arrangement of network nodes and media within an enterprise networking structure.

tunnel—Secure communications path between two peers, such as two routers.

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